This invention relates generally to electronic memories and particularly to electronic memories that use phase change material.
Phase change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure. Generally any phase change material may be utilized. In some embodiments, however, thin-film chalcogenide alloy materials may be particularly suitable.
The phase change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter, or vice versa, in response to temperature changes. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states. The phase change may be induced by resistive heating.
In some embodiments, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible, allowing the storage of multiple bits of data in a single cell.
A variety of phase change alloys are known. Generally, chalcogenide alloys contain one or more elements from Column VI of the periodic table. One particularly suitable group of alloys is the GeSbTe alloys.
A phase change material may be formed within a passage or pore through an insulator. The phase change material may be coupled to upper and lower electrodes on either end of the pore.
One problem that arises is that the adherence between the insulator and the phase change material may be poor. One solution to this problem is to provide an interfacial layer that promotes adhesion between the insulator and the phase change material. Generally, suitable interfacial layers are conductors such as titanium.
In particular, because of the use of extended lengths of phase change material, the possibility of separation arises. The use of column stripes of phase change material may require adhesion along long stripes despite the thermal expansion and contraction from subsequent processing steps. There is also accumulative stress along the column line from the phase change material stack itself and from subsequent thin-film depositions required as part of integration into an integrated circuit process flow.
Alternatively, a glue layer may be positioned between the insulator and the phase change material. However, the glue layer may degrade the phase change material or add processing cost.
Another issue with existing phase change memories is upwardly directed heat loss through the cell. The more the heat loss, the greater the programming current that is required since heat is utilized to induce the programming phase change.
Still another problem is the incorporation of species from the upper electrode into the phase change material. Species incorporation can have detrimental effects on programming properties of the phase change material.
Yet another issue with existing phase change material memories is the need for dry etching of the phase change material. The dry etch of a phase change material stack is a complicated process. Issues of undercut and re-entrant profiles may be encountered.
Thus, there is a need for better designs for phase change memories that may be manufactured using more advantageous techniques.